ox-latex: Support changable TeX compilers

ox-latex: Support changable TeX compilers

* org.el (org-latex-default-packages-alist): Only use inputenc
and fontenc in pdftex.
* ox-latex.el (latex): Add :latex-compiler.
(org-latex-compiler): New defcustom.
(org-latex-compilers): New defconst.
(org-latex--remove-packages): New function.
(org-latex-template): Use new function and variables.
(org-latex-pdf-process): Update to use %latex shorthand.
(org-latex-compile): Check and use intended compiler.
* org.texi (@LaTeX{} and PDF export): Document changes.
* ORG-NEWS: Add entry.

Adds #+latex_compiler: keyword.

Note, rubber is dropped from `org-latex-pdf-process' since it does not
allow arbitrary latex commands.

Suggested-by: Suvayu Ali <fatkasuvayu+linux@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/98921>